Today's computing devices integration level is getting higher and higher with every generation of processing devices. The internal communication speed of such devices is pushing the physical limits to a maximum. As an alternative for an electrical signal-based communication, light of photon-based chip internal communication infrastructures are envisioned—and in some cases already practically used—to increase the bandwidth of chip-internal communication systems. Light as a communication medium may be used within one single semiconductor device layer or—in 3D chip designs—also as a means of communication from one layer to another layer of the same integrated circuit. In particular, for a communication within one single semiconductor device layer, low-loss surface plasmons might be foreseen as an alternative to photonic modes for short range (<100 um) communications as properties such as speed and confinement might outweigh the high optical absorption associated with the plasmonic waveguide. In such a scheme, plasmonic lasers may be instrumental as a candidate for a radiation-source in large-scale integrated (LSI) circuits using alternatives to pure electric communications aids.
Very recently, ultra-small lasers based on collective charge of selectors at the interface between the metal and a semiconductor, called surface plasmon polaritons (SPPs), have been proposed and experimentally demonstrated. However, the strong mode confinement of SPP's modes at the semiconductor-metal interface strongly reduces the overlap of active gain material with a propagating plasmon, while the presence of metal adds to the optical losses and a large fraction of bulk semiconductor material typically do not contribute to stimulated emission. The consequent increase of the lasing threshold together with inferencing plasmon propagation losses has rendered plasmon lasers inefficient compared to their photonic counterparts.
Photonic lasers, based on semiconductor quantum well structures, have already been proposed in the 1970s and have become one of the most important semiconductor laser technologies today. Although, the overlap of the optical modes with the quantum well gain material is reduced compared to bulk semiconductors, the high gain and temperature stability provided by quantum well gain material typically over-compensate these intrinsic current losses. A main limitation for achieving integration densities of photonic components compared to micro-electronics is the much larger size of photonic devices. A conventional semiconductor laser is typical of dimensions in the order of 100s or micrometers, which is about 10,000× greater than a typical electronic MOSFET switch in an advanced computing node. The ability to scale photonic components is limited by the diffraction of light, by dimension less than the wavelength of light in a given material; the optical mode will leak out.
Thus, there may be a need for VLSI chips to have a non-electric communication mechanism that requires less space than typical light-based photonic concepts.